1. Field of the Invention
This invention relates to a so-called coated magnetic recording medium whose magnetic film is composed of two layers laid one over the other.
2. Prior Art
Audio tapes, video tapes, data-cartridges for backup, and floppy disks have been used widely as magnetic recording media. Although application of magnetic metal film by impregnation has been proposed and put to use for part of such recording media, conventional metal-coated magnetic recording media still occupy the main position, because they require only a low cost for production. The coated magnetic recording medium generally has a structure wherein a magnetic layer consisting mainly of ferromagnetic powder and a binder is spread over a non-magnetic substrate made of polyester or the like. The magnetic layer is formed after the ferromagnetic powder and binder have been allowed to disperse in an organic solvent, and the resulting paint has been spread over the non-magnetic substrate.
For a magnetic recording medium to work properly, it must be furnished with a number of properties; high electromagnetic conversion, excellent running performance and high durability. Let's take, as an example, an audiotape which is required of reproducing original sounds faithfully. To attain such objective, it must have an excellent magneto-electrical conversion, particularly it must maintain a wide separation (dynamic range) between noises and outputs over a whole audible range extending from low to high frequency regions.
Conventional methods for enlarging the dynamic range mainly consist of suppressing noises by reducing the size of ferromagnetic powder particles. However, if ferromagnetic powder particles are made finer, they will be slower in dispersion and penetration, which results in reduction of outputs. Thus, with these methods, a sufficient enlargement of dynamic ranges was impossible.
In view of this, a magnetic film with a double-layered structure has been proposed where the superficial layer is made to have a bigger coercive force than the underlying layer, or alternatively a similar structure where the superficial layer which is more conductive to noise production is made to have finer magnetic particles than the underlying layer. These means allow the product to have a wider dynamic range because they ensure high outputs and low noises at the same time.
With the magnetic recording media produced by the lastly mentioned methods, however, the superficial and underlying layers have different bias characteristics, and thus the output becomes deteriorated in an intermediate range extending from 1 to 5 kHz. With the product by those methods, therefore, it is impossible to have a wide dynamic range over a whole audible range including low and high frequency regions, namely, the product can not give a substantially linear frequency response over a whole audible range.
In view of the foregoing, the present invention intends to provide a magnetic recording medium capable of giving a substantially linear response over a whole audible frequency range.
The present inventors, to attain the object, had studied hard, found that, if, with a magnetic film having a double-layered structure, difference between the coercive forces of ferromagnetic powders contained in the superficial and underlying layers is properly chosen, the resulting magnetic recording medium will give a flat response over a whole audible frequency range, and achieved this invention.